The present invention relates to ion guides for use in mass spectrometry. The apparatus and methods for ionization described herein are enhancements of the techniques referred to in the literature relating to mass spectrometry—an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three main steps—formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means and methods exist in the field of mass spectrometry to perform each of these three functions. The particular combination of the means and methods used in a given mass spectrometer determine the characteristics of that instrument.
To mass analyze ions, for example, one might use magnetic (B) or electrostatic (E) analysis, wherein ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field, the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers. The analyzer which accepts ions from the ion guide described here may be any of a variety of these.
Before mass analysis can begin, gas phase ions must be formed from a sample material. If the sample material is sufficiently volatile, ions may be formed by electron ionization (EI) or chemical ionization (CI) of the gas phase sample molecules. Alternatively, for solid samples (e.g., semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Further, Secondary Ion Mass Spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules, resulting in the fragmentation of fragile molecules. This fragmentation is undesirable in that information regarding the original composition of the sample (e.g., the molecular weight of sample molecules) will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. (R. D. Macfarlane, R. P. Skowronski, D. F. Torgerson, Biochem. Biophys. Res Commoun. 60 (1974) 616)(“McFarlane”). Macfarlane discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules. However, unlike SIMS, the PD process also results in the desorption of larger, more labile species (e.g., insulin and other protein molecules).
Additionally, lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, Cotter et al. (R. B. VanBreeman, M. Snow, R. J. Cotter, Int. J Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Tabet, J. C., Anal. Chem. 56 (1984) 1662; or R. J. Cotter, P. Demirev, I. Lys, J. K. Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter et al., R. J., Anal. Instrument. 16 (1987) 93). Cotter modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T. Yoshica, Rapid Commun. Mass Spectrom. 2 (1988) 151 and M. Karas, F. Hillenkamp, Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. This process (i.e., MALDI) is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Further, Atmospheric Pressure Ionization (API) includes a number of ion production means and methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. This method allows for very large ions to be formed. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
In addition to ESI, many other ion production methods might be used at atmospheric or elevated pressure. For example, MALDI has recently been adapted by Laiko et al. to work at atmospheric pressure (Victor Laiko and Alma Burlingame, “Atmospheric Pressure Matrix Assisted Laser Desorption”, U.S. Pat. No. 5,965,884, and Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4.sup.th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+Collisional Cooling, poster #1272, 4.sup.th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998; and Orthogonal Injection TOFMS Anal Chem. 71(13), 452A (1999)). The benefit of adapting ion sources in this manner is that the ion optics (i.e., the electrode structure and operation) in the mass analyzer and mass spectral results obtained are largely independent of the ion production method used.
The elevated pressure MALDI source disclosed by Standing differs from what is disclosed by Laiko et al. Specifically, Laiko et al. disclose a source intended to operate at substantially atmospheric pressure. In contrast, as depicted in FIG. 1, the source 1 disclosed by Standing et al. is intended to operate at a pressure of about 70 mtorr. In addition, as shown in FIG. 1, the MALDI sample resides on the tip 6 of a MALDI probe 2 in the second pumping stage 3 immediately in front of the first of two quadrupole ion guides 4. Using a laser 7, ions are desorbed from the MALDI sample directly into 70 mtorr of gas and are immediately drawn into the ion guides 4 by the application of an electrostatic field. Even though this approach requires that one insert the sample into the vacuum system, it has the advantage of improved ion transmission efficiency over that of the Laiko source. That is, the possible loss of ions during transmission from the elevated pressure source 1, operated at atmospheric pressure, to the third pumping region and the ion guide therein is avoided because the ions are generated directly in the second pumping stage.
Elevated pressure (i.e., elevated relative to the pressure of the mass analyzer) and atmospheric pressure ion sources always have an ion production region, wherein ions are produced, and an ion transfer region, wherein ions are transferred through differential pumping stages and into the mass analyzer. Generally, mass analyzers operate in a vacuum between 10.sup.−4 and 10.sup.−10 torr depending on the type of mass analyzer used. When using, for example, an ESI or elevated pressure MALDI source, ions are formed and initially reside in a high pressure region of “carrier” gas. In order for the gas phase ions to enter the mass analyzer, the ions must be separated from the carrier gas and transported through the single or multiple vacuum stages.
As a result, the use of multipole ion guides has been shown to be an effective means of transporting ions through a vacuum system. Publications by Olivers et al. (Anal. Chem, Vol. 59, p. 1230-1232, 1987), Smith et al. (Anal. Chem. Vol. 60, p. 436-441, 1988) and Douglas et al. (U.S. Pat. No. 4,963,736) have reported the use of AC-only quadrupole ion guides to transport ions from an API source to a mass analyzer.
In the prior art, according to Douglas et al., as depicted in FIG. 2, ionization chamber 17 is connected to curtain gas chamber 24 via opening 18 in curtain gas plate 23. Curtain gas chamber 24 is connected by orifice 25 of orifice plate 29 to first vacuum chamber 44 that is pumped by vacuum pump 31. Vacuum chamber 44 contains a set of four AC-only quadrupole mass spectrometer rods 33. Also, the vacuum chamber 44 is connected by interchamber orifice 35 in separator plate 37 to a second vacuum chamber 51 pumped by vacuum pump 39. Chamber 51 contains a set of four standard quadrupole mass spectrometer rods 41.
An inert curtain gas, such as nitrogen, argon or carbon dioxide, is supplied via a curtain gas source 43 and duct 45 to the curtain gas chamber 24. (Dry air may also be used in some cases.) The curtain gas flows through orifice 25 into the first vacuum chamber 44 and also flows into the ionization chamber 17 to prevent air and contaminants in chamber 17 from entering the vacuum system. Excess sample, and curtain gas, leave the ionization chamber 17 via outlet 47.
Ions produced in the ionization chamber 17 are drifted by appropriate DC potentials on plates 23 and 29 and on the AC-only rod set 33 through opening 18 and orifice 25, and then are guided through the AC-only rod set 33 and interchamber orifice 35 into the rod set 41. An AC RF voltage (typically at a frequency of about 1 Megahertz) is applied between the rods of rod set 33, as is well known, to permit rod set 33 to perform its guiding and focusing function. Both DC and AC RF voltages are applied between the rods of rod set 41, so that rod set 41 performs its normal function as a mass filter, allowing only ions of selected mass to charge ratio to pass therethrough for detection by ion detector 49.
Douglas et al. found that under appropriate operating conditions, an increase, in the gas pressure in the first vacuum chamber 44 not only failed to cause a decrease in the ion signal transmitted through orifice 35, but in fact most unexpectedly caused a considerable increase in the transmitted ion signal. In addition, under appropriate operating conditions, it was found that the energy spread of the transmitted ions was substantially reduced, thereby greatly improving the ease of analysis of the transmitted ion signal. The particular “appropriate operating conditions” disclosed by Douglas et al maintain the second vacuum chamber 51 at low pressure (e.g. 0.02 millitorr or less) but the product of the pressure in the first chamber 44 and the length of the AC-only rods 33 is held above 2.25.times.10.sup.−2 torr-cm, preferably between 6.times.10.sup.−2 and 15.times.10.sup.−2 torr-cm, and the DC voltage between the inlet plate 29 and the AC-only rods 33 is kept low (e.g., between 1 and 30 volts) preferably between 1 and 10 volts.
As shown in FIG. 3, mass spectrometers similar to that of Whitehouse et al. (“Multipole Ion Guide for Mass Spectrometry”, U.S. Pat. No. 5,652,427) use multipole RF ion guides 42 to transfer ions from one pressure region 30 to another 34 in a differentially pumped system. In this ion source, ions are produced by ESI or APCI at substantially atmospheric pressure. These ions are transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary 60. Further, ions are transferred from this first pumping region 30 to a second pumping region 32 through a “skimmer” 56 by gas flow as well as an electric field present between these regions. Multipole ion guide 42 in the second differentially pumped region 32 accepts ions of a selected mass/charge (m/z) ratio and guides them through a restriction and into a third differentially pumped region 34 by applying AC and DC voltages to the individual poles of the ion guide 42.
Further, as depicted in FIG. 3, a four vacuum stage ESI-reflectron-TOF mass spectrometer, according to Whitehouse et al., incorporates a multipole ion guide 42 beginning in one vacuum pumping stage 32 and extending contiguously into an adjacent pumping stage 34. As shown here, ions are formed from sample solution by an electrospray process. Sample bearing liquid is introduced through the electrospray needle 26 and is electrosprayed or nebulization-assisted electrosprayed into chamber 28 as it exits the needle tip 27 producing charged droplets. The charged droplets evaporate and desorb gas phase ions both in chamber 28 and as they are swept into the vacuum system through the annulus 38 in capillary 60. According to the prior art system shown in FIG. 3, capillary 60 is used to transport ions from chamber 28, where the ions are formed, to first pumping region 30. A portion of the ions that enter the first vacuum stage 30 through the capillary exit 40 are focused through the orifice 58 in skimmer 56 with the help of lens 62 and the potential set on the capillary exit 40. Ions passing through orifice 58 enter the multipole ion guide 42, which begins in vacuum pumping stage 32 and extends unbroken into vacuum stage 34. According to Whitehouse et al. the RF only ion guide 42 is a hexapole. The electrode rods of such prior art multipole ion guides are positioned parallel and are equally spaced at a common radius from the centerline of the ion guide. A high voltage RF potential is applied to the electrode rods of the ion guide so as to push the ions toward the centerline of the ion guide. Ions with a m/z ratio that fall within the ion guide stability window established by the applied voltages have stable trajectories within the ion guide's internal volume bounded by the evenly-spaced, parallel rods. This is true for quadrupoles, hexapoles, octapoles, or any other multipole used to guide ions. As previously disclosed by Douglas et al., operating the ion guide in an appropriate pressure range results in improved ion transmission efficiency.
Whitehouse et al. further disclose that collisions with the gas reduces the ion kinetic energy to that of the gas (i.e., room temperature). This hexapole ion guide 42 is intended to provide for the efficient transport of ions from one location (i.e., the entrance 58 of skimmer 56) to a second location (i.e., orifice 50). Of particular note is that a single contiguous multipole 42 resides in more than one differential pumping stage and guides ions through the pumping restriction between them. Compared to other prior art designs, this offers improved ion transmission through pumping restrictions.
If the multipole ion guide AC and DC voltages are set to pass ions falling within a range of m/z then ions within that range that enter the multipole ion guide 42 will exit at 46 and be focused with exit lens 48 through the TOF analyzer entrance orifice 50. The primary ion beam 82 passes between electrostatic lenses 64 and 68 that are located in the fourth pumping stage 36. The relative voltages on lenses 64, 68 and 70 are pulsed so that a portion of the ion beam 82 falling in between lenses 64 and 68 is ejected as a packet through grid lens 70 and accelerated down flight tube 80. The ions are steered by x and y lens sets diagrammatically illustrated by 72 as they continue moving down flight tube 80. As shown in this illustrative configuration, the ion packet is reflected through a reflectron or ion mirror 78, steered again by x and y lens sets illustrated by 76 and detected at detector 74. As a pulsed ion packet proceeds down flight tube 80, ions with different m/z separate in space due to their velocity differences and arrive at the detector at different times. Moreover, the use of orthogonal pulsing in an API/TOF system helps to reduce the ion energy spread of the initial ion packet allowing for the achievement of higher resolution and sensitivity.
In U.S. Pat. No. 6,011,259 Whitehouse et al. also disclose trapping ions in a multipole ion guide and subsequently releasing them to a TOF mass analyzer. In addition, Whitehouse et al. disclose ion selection in such a multipole ion guide, collision induced dissociation of selected ions, and release of the fragment ions thus produced to the TOF mass analyzer. Further, the use of two or more ion guides in consecutive vacuum pumping stages allowing for different DC and RF values is also disclosed by Whitehouse et al. However, losses in ion transmission efficiency may occur in the region of static voltage lenses between ion guides. For example, a commercially available API/MS instrument manufactured by Hewlett Packard incorporates two skimmers and an ion guide. An interstage port (also called a drag stage port) is used to pump the region between the skimmers. That is, an additional pumping stage/region is added without the addition of an extra turbo pump, thereby improving pumping efficiency. In this dual skimmer design, there is no ion focusing device between skimmers, therefore ion losses may occur as the gases are pumped away. A second example is demonstrated by a commercially available API/MS instrument manufactured by Finnigan which applies an electrostatic lens between capillary and skimmer to focus the ion beam. Due to a narrow mass range of the static lens, the instrument may need to scan the voltage to optimize the ion transmission.
According to Thomson et al. (entitled “Quadrupole with Axial DC Field”, U.S. Pat. No. 6,111,250), a quadrupole mass spectrometer contains four rod sets, referred to as Q0, Q1, Q2 and Q3. A rod set is constructed to create an axial field (e.g., a DC axial field) thereon. The axial field can be created by tapering the rods, or arranging the rods at angles with respect to each other, or segmenting the rods as depicted in FIG. 4. When the axial field is applied to Q0 in a tandem quadrupole set, it speeds passage of ions through Q0 and reduces delay caused by the need to refill Q0 with ions when jumping from low to high mass in Q1. When used as collision cell Q2, the axial field reduces the delay needed for daughter ions to drain out of Q2. The axial field can also be used to help dissociate ions in Q2, either by driving the ions forwardly against the collision gas, or by oscillating the ions axially within the collision cell.
One such prior art device disclosed by Thomson et al. is depicted in FIG. 4, which shows a quadrupole rod set 96 consisting of two pair of parallel cylindrical rod sets 96A and 96B arranged in the usual fashion but divided longitudinally into six segments 96A 1 to 96A 6 and 96B 1 to 96B 6. The gap 98 between adjacent segments or sections is very small (e.g., about 0.5 mm). Each A section and each B section is supplied with the same RF voltage from RF generator, via isolating capacitors C3, but each is supplied with a different DC voltage V1 to V6 via resistors R1 to R6. Thus, sections 96A 1, 96B 1 receive voltage V1, sections 96A 2, 96B 2 receive voltage V2, and so on. This produces a stepped voltage along the central longitudinal axis 100 of the rod set 96. Connection of the R C network and thus the voltage applied to sections 96B 1 to 96B 6 are not separately shown. The separate potentials can be generated by separate DC power supplies for each section or by one power supply with a resistive divider network to supply each section. The step wise potential produce˜ an approximately constant axial field. While more sections over the same length will produce a finer step size and a closer approximation to a linear axial field, it is found that using six sections as shown produces good results.
For example, such a segmented quadrupole was used to transmit ions from an atmospheric pressure ion source into a downstream mass analyzer. The pressure in the quadrupole was 8.0 millitorr. Thomson et al. found that at high pressure without an axial field the ions of a normal RF quadrupole at high pressure without an axial field can require several tens of milliseconds to reach a steady state signal. However, with the use of an axial field that keeps the ions moving through the segmented quadrupole, the recovery or fill-up time of segmented quadrupoles, after a large change in RF voltage, is much shorter.
In a similar manner Wilcox et al. (B. E. Wilcox, J. P. Quinn, M. R. Emmett, C. L. Hendrickson, and A. Marshall, Proceedings of the 50.sup.th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Fla., Jun. 2-6, 2002) demonstrated the use of a pulsed electric field to eject ions from an octapole ion guide. Wilcox et al. found that the axial electric field caused ions in the octapole to be ejected more quickly. This resulted in an increase in the effective efficiency of transfer of ions from the octapole to their mass analyzer by as much as a factor of 14.
Another type of prior art ion guide, depicted in FIG. 5, is disclosed by Franzen et al. in U.S. Pat. No. 5,572,035, entitled “Method and Device for the Reflection of Charged Particles on Surfaces”. According to Franzen et al., the ion guide 11 comprises a series of parallel rings 12, each ring having a phase opposite that of its two neighboring rings. Thus, along the axis there exists a slightly undulating structure of the pseudo potential, slightly obstructive for a good and smooth guidance of ions. On the other hand, the diffuse reflection of particles at the cylinder wall is favorable for a fast thermalization of the ion's kinetic energy if the ions are shot about axially into the cylinder. This arrangement generates, in each of the ring centers, the well known potential distribution of ion traps with their characteristic equipotential surfaces crossing in the center with angles of a=2 arctan(1/20.5). The quadropole fields, however, are restricted to very small areas around each center. In the direction of the cylinder axis; the pseudo potential wells of the centers are shallow because the traps follow each other in narrow sequence. In general, the pseudo potential wells are less deep the closer the rings are together. Emptying this type of ion guide by simply letting the ions flow out leaves some ions behind in the shallow wells.
In this prior art ion guide according to Franzen, an axial DC field is used to drive the ions out, ensuring that the ion guide is completely emptied. The electric circuits needed to generate this DC field are shown in FIG. 5. As shown, the RF voltage is supplied to the ring electrodes 12 via condensers, and the rings are connected by a series of resistance chokes 14 forming a resistive voltage divider for the DC voltage, and hindering the RF from flowing through the voltage divider. The DC current is switchable, and the DC field helps to empty the device of any stored ions. With rings 12 being approximately five millimeters in diameter, resistance chokes 14 of 10 microhenries and 100 Ohms, and capacitors 16 of 100 picofarads build up the desired DC fields. Fields of a few volts per centimeter are sufficient.
A similar means for guiding ions at “near atmospheric” pressures (i.e., pressures between 10.sup.−1 millibar and 1 bar) is disclosed by Smith et al. in U.S. Pat. No. 6,107,628, entitled “Method and Apparatus for Directing Ions and Other Charged Particles Generated at Near Atmospheric Pressures into a Region Under Vacuum”. One embodiment, illustrated in FIG. 6, consists of a plurality of elements, or rings 13, each element having an aperture, defined by the ring inner surface 20. At some location in the series of elements, each adjacent aperture has a smaller diameter than the previous aperture, the aggregate of the apertures thus forming a “funnel” shape, otherwise known as an ion funnel. The ion funnel thus has an entry, corresponding with the largest aperture 21, and an exit, corresponding with the smallest aperture 22. According to Smith et al., the rings 13 containing apertures 20 may be formed of any sufficiently conducting material. Preferably, the apertures are formed as a series of conducting rings, each ring having an aperture smaller than the aperture of the previous ring. Further, an RF voltage is applied to each of the successive elements so that the RF voltages of each successive element is 180 degrees out of phase with the adjacent element(s), although other relationships for the applied RF field would likely be appropriate. Under this embodiment, a DC electrical field is created using a power supply and a resistor chain to supply the desired and sufficient voltage to each element to create the desired net motion of ions through the funnel.
Each of the ion guide devices mentioned above in the prior art have their own particular advantages and disadvantages. For example, the “ion funnel” disclosed by Smith et al. has the advantage that it can efficiently transmit ions through a relatively high pressure region (i.e., >0.1 mbar) of a vacuum system, whereas multipole ion guides perform poorly at such pressures. However, the ion funnel disclosed by Smith et al. performs poorly at lower pressures where multipole ion guides transmit ions efficiently. In addition, this ion funnel has a narrow range of effective geometries. That is, the thickness of the plates and the gap between the plates must be relatively small compared to the size of the aperture in the plate. Otherwise, ions may get trapped in electrodynamic “wells” in the funnel and therefore not be efficiently transmitted.
Similarly, the ion guide disclosed by Franzen et al. and shown in FIG. 5 must have apertures which are large relative to plate thickness and gap. Also while Franzen et al.'s ion guide can have an “axial” DC electric field to push the ions towards the exit, the DC field cannot be changed rapidly or switched on or off quickly. That is, the speed with which the DC field is switched must be much slower than that represented by the frequency of the RF potential applied to confine the ions. Similarly, the segmented quadrupole of Thomson et al. allows for an axial DC electric field. However, in Thomson et al., the field cannot be rapidly switched.
As discussed below, the ion guide according to the present invention overcomes many of the limitations of prior art ion guides. The ion guide disclosed herein provides a unique combination of attributes making it more suitable for use in the transport of ions from high pressure ion production regions to low pressure mass analyzers.